1. Field of the Invention
The present invention is generally directed to a method and apparatus for bonding spacers to conductors, such as hollow conductors used in high power transmission lines, whereby conductor segments are bonded together to form long transmission lines.
2. State of the Art
The efficiency of high voltage power transmission lines has been recently improved by using gas insulated cable line systems to replace conventional cable systems and overhead lines. Gas insulated cable systems are formed from hollow inner conductors suspended in a hollow conductive sheath. The hollow inner conductors are formed as interconnectable segments, whose sheaths can also be bonded together, to form a relatively long distance transmission line. A conductive transmission line system configured in this manner is the compressed gas insulated transmission (CGIT) bus system, available from Asea Brown Boveri (ABB) and described in the ABB product brochure entitled "Gas Insulated Transmission Bus".
An exemplary embodiment of a conductive transmission line configured as a gas insulated cable is illustrated in FIG. 1A. FIG. 1A shows a cross section of a hollow inner conductor 102 formed of a conductive material, such as an aluminum alloy. A sheath is formed as an outer conductor 104 that is concentrically located about the hollow inner conductor 102. Typically, the hollow inner conductor 102 is used to provide high voltage power transmission, while the outer conductor 104 is grounded to electrically shield the inner conductor 102. Both the hollow inner conductor 102 and the outer conductor 104 are formed in segments of predetermined lengths, such as 18 meter lengths. Both the hollow inner conductor 102 and the outer conductor 104 can be formed of any known conductive material, such as aluminum or aluminum alloys (e.g., aluminum/tin, or aluminum/magnesium).
To maintain high efficiency power transmission, the outer conductor 104 is spaced equidistant from the hollow inner conductor 102 at all locations about the periphery of the hollow inner conductor 102. Devices are provided on the periphery of the hollow inner conductor 102 at predetermined distances along its length to maintain a fixed separation distance between the outer periphery of the inner conductor 102 and the inner periphery of the outer conductor 104.
For example, FIG. 1A shows a cross-sectional view of a bracket formed as posts 110 located on a sleeve 108. The sleeve 108 is slid over the outer periphery of the hollow inner conductor 102 to a predetermined location along a length of the inner conductor. The sleeve 108 can be welded to the outer periphery of the hollow inner conductor 102. The bracket can be formed with any number (e.g., 3) of posts 110 extending from the sleeve toward an inner periphery of the outer conductor 104. The posts can be formed of any material including but not limited to filled epoxy (e.g., biphenyl epoxy). An outer end of each post 110 is typically formed with a movable element 112, such as a ball bearing, wheel or the like. After attachment of the sleeve 108 to an outer periphery of the hollow inner conductor 102, the outer conductor 104 can be slid into a concentric position about the hollow inner conductor 102, with the movable elements 112 serving as guides for maintaining an interior of the outer conductor 104 equidistant from an outer surface of the hollow inner conductor 102.
FIG. 1B shows a cross-sectional view of the FIG. 1A conductive transmission line along its longitudinal length. As shown therein, each of multiple sleeves 108 have plural posts 110. The sleeves are spaced equidistant from one another along a longitudinal length of a segment 118 of the conductive transmission line. In the exemplary FIG. 1B illustration, note that ends of the segment 118 are not configured with sleeves 108 having posts 110. Rather, a sleeve 108 which is mounted at either or both ends of segment 118 can be formed with cone 114 about its periphery. The cone is included in selected ends of selected segments 118 because the area 116 between an outer surface of the inner conductor 102 and an inner surface of the outer conductor 104, representing a gas compartment, is filled with a gas dielectric, such as sulpherhexofluoride (e.g., SF.sub.6) nitrogen-sulpherhexofluoride mixture (e.g., N.sub.2 SF.sub.6) or any other gas dielectric. The cone 114 serves as a gas barrier to prevent the gas dielectric included in one segment from leaking upon rupture of another segment of the conductive transmission line. The cones can be formed in a known manner using, for example, an epoxy with glass filler (e.g., silicone dioxide (SiO.sub.2)).
As those skilled in the art will appreciate, although a conductive transmission line system such as that illustrated in FIGS. 1A and 1B provides relatively efficient transmission of power, a significant cost of these conductive transmission lines can be directly attributed to their installation in the field. More particularly, to provide flexibility during installation with respect to the arrangement of these conductive transmission lines, they are sent from a factory to the field with the hollow inner conductor and the outer conductor being provided separate of one another. In addition, the sleeves are not mounted to the inner conductor. That is, although the posts 110 are preattached to the sleeves 108, the sleeves 108 are not mounted to the hollow inner conductor 102 in the factory, but rather this mounting is implemented in the field.
These conductive transmission lines, when assembled, can traverse several hundred kilometers or more in length. The transmission lines are often installed above ground in cable trays or below ground by first trenching the earth in the place where the system is to be laid. The sleeves 108 having posts 110 and cones 114 are then welded to an outer periphery of the inner conductor, at predetermined separation distances along a length of the inner conductor 102. Afterwards, the outer conductor 104 is placed concentrically about the inner conductor 102 and sleeves 108. Once a given segment has been assembled, the segment is attached to another segment which has been assembled in similar fashion. This process is repeated until a desired length of the conductive transmission line has been assembled and installed.
In the foregoing assembly process, the sleeves 108 are welded in the field to the outer surface of the inner conductor 102. The welding of the cylindrical sleeves 108 to the outer surface of the hollow inner conductor 102 involves a 360.degree. weld about the periphery on both ends of the sleeve. The entire inner conductor, the sleeves 108 and posts 110 must then be carefully examined to remove any imperfections due to the welding process.
The high percentage cost of installing the FIG. 1A system can be attributed to the lack of a controlled environment in the field, because this results in a higher number of imperfections. The removal of imperfections involves having a skilled assembler manually examine all outer surfaces of the segment to detect any high or low points which occurred as a result of welding. The assembler must machine and polish all surfaces to remove these imperfections (e.g., remove any high points using a lathe). During the welding process, solder can splatter onto the brackets, sleeves and exterior surfaces of the inner conductor to create imperfections which can significantly detract from the efficiency of the conductive transmission line and which must be removed. As those skilled in the art will appreciate, any imperfections which are not detected and addressed can stress the transmission line at that location, and can lead to shut down of the entire conductive transmission line. This, of course, results in significant costs and aggravation to both power providers and consumers.
As can be appreciated, installing a system as described with respect to FIGS. 1A and 1B is a time consuming and expensive process. Accordingly, it would be desirable to develop an assembly process for conductive transmission lines as illustrated in FIGS. 1A and 1B which can be cost-effectively and reliably implemented in the field, without detrimentally affecting the operability and efficiency of the conductive transmission line.